1. Field of the Invention
The present invention relates to a light emitting device.
2. Discussion of the Background Art
An organic light emitting device, which is also called an organic light emitting diode (OLED), is a self-luminous device that causes a fluorescent material to emit light when electron-hole pairs are generated. Self-luminous light emitting devices have a faster response rate and a lower direct current driving voltage than passive light emitting devices such as liquid crystal displays requiring a separate light source and can be implemented using a very thin film. These advantages allow organic light emitting displays to be implemented in various configurations such as a wall mount type and a portable type.
An organic light emitting device implements colors using pixels in which sub-pixels of red, blue and green produce one color. According to driving types of sub-pixels, an organic light emitting device may be classified as a passive matrix OLED (PMOLED) that is a simple matrix and an active matrix OLED (AMOLED) that uses a thin film transistor to drive the device.
Various AMOLED driving methods have been used such as a current based driving method, a voltage based driving method and a digital driving method.
FIG. 1 illustrates an equivalent circuit diagram of an AMOLED 10 pixel based on a typical current based driving method. The AMOLED 10 is configured in a 2T1C structure including two TFTs and one capacitor. Particularly, the two TFTs are a driving TFT and a switching TFT denoted as DT and ST in FIG. 1, and the capacitor is a storage capacitor denoted as Cst in FIG. 1. The driving TFT DT and the switching TFT ST are N-channel metal oxide semiconductor (NMOS) transistors.
The AMOLED 10 includes an OLED in which an organic emissive layer is formed between charge transport layers. The OLED is connected between a supply voltage VDD and the driving TFT DT. The OLED emits light corresponding to an amount of output current IOLED supplied from the driving TFT DT.
The driving TFT DT is connected between the OLED and a ground voltage GND, and a gate of the driving TFT DT is connected with one end of the storage capacitor Cst. The driving TFT DT supplies the output current IOLED to the OLED.
The switching TFT ST is connected between the gate of the driving TFT DT and a data line 12, and a gate of the switching TFT ST is connected with a scan line 14. Therefore, when a scan signal is supplied to the gate of the switching TFT DT through the scan line 14, the switching TFT ST turns on to supply a data signal to the gate of the driving TFT DT. As a result, the data signal is stored into the storage capacitor Cst.
The storage capacitor Cst stores the data signal switched by the switching TFT ST, and this stored data signal allows the driving TFT DT to retain an ‘on’ state even if the switching TFT ST turns off by disablement of the scan signal.
The typical AMOLED 10 stores the data signal on the storage capacitor Cst and drives the driving TFT DT in response to the stored data signal to make the OLED emit light using the output current IOLED corresponding to the data signal.
The typical AMOLED 10 may degrade due to various factors because the AMOLED 10 uses the driving TFT DT. Hence, as illustrated in FIG. 2, the driving TFT DT has a current-voltage characteristic curve shifted to the right. As a result, a threshold voltage Vth generally increases. For instance, the threshold voltage Vth may increase from 2 V to 2.5 V.
As the threshold voltage Vth increases, the output current IOLED of the driving TFT DT of the typical AMOLED 10 decreases. Particularly, the decrease in the output current IOLED generally reduces the brightness of the OLED. The below mathematical equation shows the above described relationship between the threshold voltage Vth and the output current IOLED.
                              I          OLED                =                              β            2                    ⁢                                    (                              Vgs                -                Vth                            )                        2                                              Eq        .                                  ⁢        1            
Herein, IOLED, β, Vgs, and Vth represent an output current of the driving TFT DT, a constant of the driving TFT DT, a voltage between a source and a gate of the driving TFT DT, and a threshold voltage of the driving TFT DT, respectively.
In the typical AMOLED 10, brightness of the OLED may decrease due to the increase in the threshold voltage Vth. Thus, organic light emitting displays comprising typical AMOLEDs may have a shortened durability.